function W = gradDesc(alpha, W, P)
% 梯度下降
% input:
%     alpha - scalar, learning ratio
%     W - matrix cell array, weight
%     P - matrix cell array, partial
% output:
%     W - matrix cell array, weight
    for k = 1: length(W)
        W{k} = W{k} - alpha * P{k};
    end
end